The production and supply of plants such as vegetables and fruits are susceptible to drops due to climatic changes, pest damage, etc. On the other hand, today, with much attention paid to safe foods and higher food self-sufficiency, stable supply of safe plants is sought. This has brought about the development of technology for growing plants on factory equipment without using sunlight.
To grow a plant on factory equipment (in a plant factory), light (a source of light) is needed as a substitute for sunlight. Nowadays, light emitting elements such as LEDs (light-emitting diodes) are used as light sources in lighting devices for plant growth.
As is generally known, for efficient growth of plants in a plant factory, light having peak wavelengths in a plurality of wavelength regions is effective (peak wavelength denoting a wavelength at which a maximum relative radiation intensity is observed in a spectrum distribution). Superficially, effective is light having peak wavelengths in a wavelength range of 600 nm to 700 nm (light in this range is considered necessary for photosynthesis) and in a wavelength range of 400 nm to 480 nm (light in this range is considered necessary for proper morphogenesis of leaves).
Patent Document 1 identified below discloses a lighting device provided with a plurality of light emitting elements respectively having peak wavelengths in different wavelength regions. With this configuration, it is possible to irradiate a plant with light having peak wavelengths in different wavelength regions. However, provided with a plurality of light emitting elements, the configuration suffers from a complicated design of the drive circuit to accommodate the electrical and temperature characteristics of the individual light emitting elements, and from high cost resulting from the increased number of components. Moreover, a light emitting element having a peak wavelength in the wavelength range of 600 nm to 700 nm is typically fabricated by use of a compound, such as GaAlP, that is vulnerable to humidity, and thus requires some measure against humidity to be taken when used in a high-humidity environment as one suitable for plant growth.
Patent Document 2 identified below discloses a lighting device provided with a blue LED, a GGG (gadolinium-gallium-garnet) phosphor (far-red phosphor), and a red phosphor. With this configuration, it is possible to emit, as well as blue light, far-red and red light from the far-red and red phosphors, respectively, excited by excitation light from the blue LED.